Sodium Vapour Lamps

Sodium Vapour Lamps:

Sodium vapour has the highest theoretical luminous efficiency and gives monochromatic orange-yellow light. The monochromatic light makes objects look grey, on account of which these lamps are used for street and highway lighting. 

The lamp consists of a discharge tube having special composition of glass to withstand the high temperature of the electric discharge. The discharge tube is surrounded by an outer tube as shown in fig. For heating the cathode a transformer is included. Sodium below 60⁰C is in solid state. For starting the lamp the electric discharge is allowed to take place in neon gas. The temperature inside the discharge tube rises and vaporizes sodium. Operating temperature is around 300⁰C. It takes about 10 minutes for the sodium vapour to displace the red colour of the neon by its own yellow colour. The lamp takes around half an hour to reach full output. A choke is provided for stabilizing the electric discharge and a capacitor for power factor improvement. Although the theoretical efficiency is 475 lumens/watt, the practical light output is around 40-50 lumen/watt.

HPSV lamps are used for lighting of public thoroughfares, storage yards, open-air work sites, process plants, interiors with high ceiling heights, etc.